The x-ray absorption of a particular material depends on the energy of the x-ray photons directed through that material. Further, the functional relationship for x-ray absorption and x-ray energy differs between different materials.
The difference between absorption functions for different materials, that is, the difference in the relation between absorption and the x-ray energy, has been exploited to isolate the contribution of specific body materials to the absorption of x-rays along a given x-ray beam path. Quantitative measurement of the absorptions of single materials may be used to selectively image that material even if it overlaps with other materials along the direction of x-ray propagation.
This technique of selective imaging has been successfully applied, in one example, to measuring a patient's total bone mass and bone density in isolation from the patient's superimposed tissue mass. Bone density measurements are important in the treatment of bone diseases such as osteoporosis and in gauging the success of bone implants by evaluating the health of the bone in the neighborhood of the implant.
The difference in absorption functions for a given mass of two different body materials is primarily the result of two absorption mechanisms: photoelectric absorption and Compton scattering. Body materials may be distinguished by the degree to which each of these mechanisms contribute to their total x-ray absorption. It follows, also, that the absorption function for any body material may be accurately modeled by combining the functions for the photoelectric absorption and Compton scattering. This combination function is the sum of the photoelectric absorption and Compton scattering functions as weighted by coefficients unique to the particular material.
The ability to model the absorption of any material by a weighted combination of photoelectric absorption and Compton scattering is key to the selective imaging of materials. Selective imaging is accomplished by making two absorption measurements at two x-ray energies, and then, knowing in advance the coefficients associated with the body materials of interest, solving the resulting two independent equations to determine the total mass associated with each material. Although there are generally more than two body materials of interest, in many diagnostically important cases, the materials may be grouped in two categories, such as bone and tissue, which may be generally distinguished by these techniques.
A convenient way of producing the two energies of x-rays needed for selective imaging is by controlling the voltage of an x-ray tube. A typical x-ray tube consists of cathode which contains a heated filament and produces electrons which are directed toward a target anode. The cathode and anode are held in an evacuated envelope and a biasing voltage is placed across the cathode and anode to accelerate electrons toward the anode. A portion of the energy of the electrons incident on the anode is converted into x-ray radiation, the spectral content of which is dependent on the biasing voltage. Herein, the term "x-ray energy" will be used to mean the quantum energy of the photons that comprise the x-ray beam.
Accordingly, two different x-ray energies, and two different absorption measurements at those different energies, may be made simply by changing the voltage on the x-ray tube. The change in voltage across the x-ray tube also may be accompanied by the mechanical insertion of filters into the path of the x-rays. Filters preferentially absorb photons at certain x-ray energies and are used to increase the spectral separation of the two beams. Each voltage may have an associated filter or both may use the same filter.
In order to ensure that the two measurements of absorption, at the two energies of x-rays, are taken of the same portion of a patient, it is desirable that these measurements be taken in rapid succession. Otherwise, patient motion may corrupt the data. For scanning x-ray systems, where a narrow x-ray beam sweeps successively over different portions of the patient, the above condition requires that the beam of x-rays be repeatedly and rapidly switched between the two energy levels during its scan.
For each x-ray energy, a signal related to x-ray absorption is generated by a detector receiving the x-rays after they pass through the body. Common types of detectors are ionization chambers, scintillation detectors, and solid-state detectors which measure scintillations produced by the effect of x-rays passing through certain solid materials. Aside from the detection efficiency of the detector, the signal-to-noise ratio of the electrical signals produced by these detectors, and hence the quality of the selective imaging data, is predominantly a function of the flux density of the x-ray beams, that is, the number of x-ray photons per unit time per unit area at the object, which is transmitted through the object and detected.
Unfortunately, the flux density received by the detector can shift dramatically as the energy of the x-ray beam is changed. Generally, there is higher attenuation of x-rays by body materials at lower x-ray energies. Thus, the flux density of the x-rays received by the detector during the low energy portion of the measurement will be comparably reduced. This additional absorption is compounded by the fact that x-ray tubes are less efficient at lower energies and thus produce a lower flux density beam.
The signal-to-noise ratio of the resulting selective material image is a function of the signal-to-noise ratio of the measurements with each of the two beams. The measurement with the lesser signal-to-noise ratio of the high and low energy may disproportionately degrade the signal-to-noise ratio of the combined signals. Ideally the signal-to-noise ratios of the two measurements may be adjusted to optimize the quality of the final result for a given x-ray exposure to the patient. See, for example, Sorenson, Duke and Smith, Med. Phys. 16, p. 75-80, 1989.
Within the range of the sensitivity of the detector used to convert the x-ray beam to an electrical signal, the signal-to-noise ratio of the signal generated during the low energy portion of the scan may be selectively increased by increasing the flux density of the x-ray beam from the x-ray tube during that portion of the scan. This may be done, without appreciably changing the energy of the x-rays, by increasing the current flow in the x-ray tube while holding the anode to cathode voltage constant, i.e. increasing the cathode filament current and thus the cathode temperature.
For medical imaging, the benefit of increasing the flux density must be balanced by the need to reduce, to the extent possible, the total exposure of the patient to ionizing x-ray radiation. Therefore, it is desirable also, to reduce the flux density during the high energy portion of the scan. Again, this may be done by adjusting the cathode temperature, in this case by lowering the filament current.
Unfortunately, the cathode temperature may not be adjusted rapidly, and use of the cathode temperature to control the flux density of the x-ray beam prevents rapid change in beam energies as is needed to reduce patient motion problems. Slow switching speed also may significantly prolong the examination time in a scanning system and in all system results in needless exposure to the patient.
X-ray tube current can also be controlled by use of an x-ray tube with a grid and associated control circuitry. See, for example, U.S. Pat. No. 4,361,901. This significantly increases the complexity and cost of the x-ray source.